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await-dictionary

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Stage 1
Classification: API Change
Human Validated: KW
Title: Await Dictionary
Authors: Alexander J. Vincent
Champions: Ashley Claymore, Jordan Harband, Chris de Almeida
Last Presented: March 2023
Stage Upgrades:
Stage 1: 2023-03-23
Stage 2: NA
Stage 2.7: NA
Stage 3: NA
Stage 4: NA
Last Commit: 2023-03-25
Topics: async objects concurrency
Keywords: promise property wait asynchronous
GitHub Link: https://github.com/tc39/proposal-await-dictionary
GitHub Note Link: https://github.com/tc39/notes/blob/HEAD/meetings/2023-03/mar-22.md#await-dictionary-for-stage-1

Proposal Description:

Await dictionary of Promises

Status

Stage: 1

Champions:

Authors:

Motivation

await on individual properties creates a waterfall, rather than running requests in parallel:

const obj = {
  shape: await getShape(),
  color: await getColor(),
  mass: await getMass(),
};

Promise.all helps, but is based on order, rather than names, which could lead to mixups:

const [
  color,
  shape,
  mass,
] = await Promise.all([
  getShape(),
  getColor(),
  getMass(),
]);

Solutions using existing syntax can be verbose and pollute the number of variables in scope:

const shapeRequest = getShape();
const colorRequest = getColor();
const massRequest = getMass();
 
const shape = await shapeRequest;
const color = await colorRequest;
const mass = await massRequest;

Potential solutions

Promise.ownProperties

const {
  shape,
  color,
  mass,
} = await Promise.ownProperties({
  shape: getShape(),
  color: getColor(),
  mass: getMass(),
});

Promise.fromEntries

const {
  shape,
  color,
  mass,
} = await Promise.fromEntries(Object.entries({
  shape: getShape(),
  color: getColor(),
  mass: getMass(),
}));

Existing solutions

http://bluebirdjs.com/docs/api/promise.props.html

https://github.com/slorber/combine-promises

https://github.com/sindresorhus/p-props

Implementations

Polyfill/transpiler implementations

None.

Native implementations

None.

Q&A

Q: Why not a deep-copy option?

A: Deep copy traditionally has been left out of JavaScript for a number of reasons. While a recursive promise walking API might be possible, combining it with this feels overcomplicated and unlikely to pass TC39’s smell tests.

Q: Why not all keys?

A: This implies walking the prototype chain. Note this proposal would have been Promise.allProperties() originally, but that naming confuses what the polyfill does above with that intended use case. Plus, if a prototype has promises on it, then how would we safely construct the prototype of the resulting object? Going down the prototype chain opens a can of worms.

Q: What about symbol keys?

A: We haven’t really talked about them yet. That said, Object.entries(), for better or for worse, only considers string keys.


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